1. Field of the Invention
The present invention relates generally to a Frequency Division Multiple Access (FDMA) system, and in particular, to a method and apparatus for transmitting/receiving uplink pilots used for channel estimation and measurement of an uplink.
2. Description of the Related Art
The next generation mobile communication system uses Distributed Frequency Division Multiple Access (DFDMA) and Localized Frequency Division Multiple Access (LFDMA) as a useful uplink multiple access scheme. In the uplink, an increase in Peak to Average Power Ratio (PAPR) causes performance degradation due to linear characteristics of a power amplifier of a transmitter, or terminal, resulting in a reduction in cell coverage. Compared with the system using multiple carriers, DFDMA and LFDMA, as they both use a single carrier, are advantageous in that they can solve the PAPR problem.
In the wireless mobile communication system where channel characteristics undergo a change in time and frequency domains, proper pilot signals are transmitted along with data signals to make it possible to perform channel estimation necessary for demodulating the data signals. A system employing DFDMA/LFDMA generally uses a Time Division Multiplexing (TDM) format that distinguishes between data signals and pilot signals in the time domain before transmission, in order to maintain the preferred PAPR characteristic.
FIG. 1 illustrates typical TDM-formatted data signals and pilot signals.
Referring to FIG. 1, reference numeral 110 represents one time slot including one transmission time interval (TTI) or a plurality of TTIs. As the most general TDM format, a plurality of time symbols 120 having a duration of the same length of time Td exist in the one time slot 110, and each of the time symbols 120 is allocated for transmission of a pilot signal or a data signal. To prevent inter-symbol interference, a guard period (or guard interval) 130 having a length of a time interval Tg is inserted between time symbols 120.
With reference to FIGS. 2 and 3, a brief description will be made of a structure of a transmission apparatus for implementing a DFDMA/LFDMA system that transmits data signals and pilot signals in the stated-above TDM format.
FIGS. 2A to 2D illustrate a structure of a transmission apparatus for the typical DFDMA system. FIG. 2A and FIG. 2B illustrate a spectrum 200 in a frequency domain and a one-symbol signal format 210 in a time domain of the DFDMA system, respectively, and FIGS. 2C and 2D illustrate an exemplary transmission apparatus 220 in the time domain and an exemplary transmission apparatus 230 in the frequency domain of the DFDMA system, respectively.
Referring to FIG. 2A, the spectrum 200 in the frequency domain of the DFDMA system has a format in which C frequency elements 201 are spaced apart over the full band, and a set of C scattered frequency elements is called a comb 202, which is a resource allocation unit. If a distance between frequency elements in one comb 202 is defined as the number R of repetitions (hereinafter repetition R) 203, a value of the R 203 is equal to the total number of combs. If comb indexes of 1˜R are sequentially assigned to the frequency elements beginning at the position of a first frequency element where each comb starts in the full band, the comb 202 is assigned a comb index of 2.
FIG. 2B illustrates a signal format 210 of a length-Td DFDMA data symbol in the time domain. If the basic time element is defined as a sample, a length of a sample interval 211 is Ts and a reciprocal of the sample interval length is a sampling frequency. In a time-domain signal format of the DFDMA system, a block of C data symbols is repeated as many times as the repetition R defined in the spectrum 200. Because 4 data symbols of a, b, c and d are repeatedly transmitted R times for one data or pilot symbol interval Td 212, a relationship between the sample interval length Ts 211 and the data/pilot symbol interval Td 212 is given as in Equation (1).Td=C·R·TS  (1)
A structure of a transmitter for generating a DFDMA transmission signal having the signal format 210 in the time and frequency domains will be described with reference to FIGS. 2C and 2D.
Referring to FIG. 2C, a transmission apparatus 220 in the time domain receives an input bit stream using a proper bit-to-constellation mapper 221, and outputs C data symbols. Exemplary bit-to-constellation mapping methods used in the bit-to-constellation mapper 221 include Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM), etc. In addition, a pilot sequence generator 222 generates C pilot symbols.
Output symbols of the bit-to-constellation mapper 221 and the pilot sequence generator 222 are input to a selector 223, and the selector 223 selects one type of the symbols according to the current time symbol index. An output of the selector 223 is repeatedly output by a repeater 224 as many times as the repetition R, and then phase-shifted by a comb-specific phase shifter 225. A phase-shifted ith comb is expressed as in Equation (2).
                                          S            l                          (              i              )                                =                      exp            ⁡                          (                                                -                  j                                ·                l                ·                i                ·                                                      2                    ⁢                    π                                                        R                    ·                    C                                                              )                                      ,                                  ⁢                  l          =          0                ,                              …            ⁢                                                  ⁢                          R              ·              C                                -          1                                    (        2        )            
In Equation (2), l denotes a sample index described in FIG. 2B.
The signal phase-shifted by the comb-specific phase shifter 225 passes through a guard interval adder 226 to prevent inter-symbol interference, and then is transmitted over a wireless channel. The guard interval adder 226 can use any one or both of a zero-padding technique of transmitting no signal, and a cyclic prefix, as a guard interval.
FIG. 2D illustrates a transmission apparatus for implementing DFDMA that transmits data signals and pilot signals by TDM, in the frequency domain. A bit-to-constellation mapper 231, a pilot sequence generator 232 and a selector 233 of FIG. 2D are identical in operation to the bit-to-constellation mapper 221, the pilot sequence generator 222 and the selector 223 of FIG. 2C, so a description thereof will be omitted.
Referring to FIG. 2D, a pilot or data signal output from the selector 223 according to a symbol index is converted into a frequency-domain signal through a size-C Fast Fourier Transform (FFT) block 236. An output signal of the FFT block 234 is mapped to a size-C*R IFFT block 236, and mapping between the output of the FFT block 234 and the input to the IFFT block 236 is achieved by a comb-specific mapper 235. The comb-specific mapper 235 differentiates a first IFFT input index for each individual comb while maintaining an interval at which the outputs of the FFT block 234 are input to the IFFT block 236 at the above-defined repetition R, thereby mapping the outputs of the FFT block 234 such that they should not overlap for each individual comb. Because mapping between the output of the FFT block 234 and the input to the IFFT block 236 is performed in the frequency domain, it can be noted that signals for each individual comb are coincide with the DFDMA frequency spectrum 200 described in FIG. 2A. The output of the IFFT block 236, which is a time-domain signal, passes through a guard interval adder 237, and then is transmitted over a wireless channel. The guard interval adder 217 adds a guard interval to the output of the IFFT block 236 in the manner of FIG. 2C before transmission.
The frequency spectrum 200 and the signal format 210 in the time-frequency domain and the transmission apparatuses 220 and 230 of the DFDMA system has been described so far with reference to FIGS. 2A to 2D. With reference to FIG. 3, a description will now be made of LFDMA. LFDMA can also be implemented by using the above-described spectrum, signal format and transmission apparatus and properly controlling the repetition R and the comb-specific phase shifting or comb-specific mapping. In the LFDMA system, because resources of the continuous frequency domain are allocated to a terminal, a domain of the continuous frequency elements that the terminal is allocated is defined as Region. FIG. 3A illustrates a frequency-domain spectrum 310 of the LFDMA system, and FIG. 3B illustrates an LFDMA transmission apparatus 320 in the frequency domain. A bit-to-constellation mapper 321, a pilot sequence generator 322, a selector 323, an FFT block 324, an IFFT block 326, and a guard interval adder 327 of FIG. 3B are equal in operation to the elements of FIG. 2C, so a description thereof will be omitted.
Because the frequency-domain spectrum 310 of the LFDMA system appears in the continuous frequency domain, it can be noted that a value of the repetition R is 1 and Region occupies a frequency range 312 including a set of C adjacent frequency elements. Region allocated to an ith terminal is distinguished according to initial start frequency Φ(i) 311. In addition, a range of the total frequency band includes Ctotal 313 frequency elements.
In the LFDMA transmission apparatus 320 of FIG. 3B, a comb-specific mapper 325 continuously maps outputs of an FFT block 324 to C input nodes in sequence beginning at a Φ(i)th input node of an IFFT block 326. Implementation of the LFDMA transmission apparatus 320 is possible by properly modifying mapping parameters for the transmission apparatus 230 in the frequency domain of the DFDMA system in this way. In this case, a size of the IFFT block 326 is Ctotal corresponding to the total frequency band.
Similarly, it is also possible to implement the transmission apparatus of the LFDMA system by using the transmission apparatus 220 in the time domain of the DFDMA system. The apparatus has the structure shown in FIG. 2C, but the repetition R is 1 and the phase shifted by the comb-specific phase shifter 225 is set in accordance with Equation (3) below. In Equation (3), Φ(i) means an index of a frequency element where an ith Region starts in the full band.
                              p          l                      (            i            )                          =                  exp          ⁡                      (                                          -                j                            ·              l              ·                              ϕ                ⁡                                  (                  i                  )                                            ·                                                2                  ⁢                  π                                                  C                  total                                                      )                                              (        3        )            
In Equation (3), a sample index l is an integer between 0 and RC−1.
Although the transmitter structure for one comb or Region has been described above, when a plurality of combs or Regions are used, extension to a transmission apparatus for multiple combs or Regions is possible by summing up the signals generated using a plurality of transmission apparatuses. Because this extension is obvious to those skilled in the art, it will be assumed in the following description that one terminal uses one comb/Region, for convenience.
The basic TDM pilot signal format means a format in which a plurality of time symbols having a length of the same time duration Td exist in one time slot as described in FIG. 1, and each of the time symbols is allocated for transmission of pilot or data. However, when a channel impulse response characteristic suffers a change even in one time slot due to the high moving velocity of the terminal, simply using one pilot symbol in one time slot is not sufficient to accurately estimate and measure channel characteristics. If several pilot symbols are used to solve this problem, pilot overhead increases resulting in a decrease in data transmission efficiency. Therefore, there is a need for a scheme for efficiently transmitting uplink pilots in a fast-varying channel environment without increasing the pilot overhead.